In radio transmitters, a transmission signal, i.e. the signal being transmitted, is amplified in a radio frequency power amplifier which amplifies the transmission signal to a level suitable for transmission over an air interface to a radio receiver. The level of the power-amplified transmission signal should be high enough to enable the radio receiver to decode information contained in the transmission signal.
In a polar transmitter structure, the transmission signal is separated into an amplitude component and phase component. The phase component is up-converted to a radio frequency and then applied to an input node of the power amplifier. The amplitude component is applied to a power supply signal path in the transmitter and used to provide the power amplifier with a power supply signal.
Components in the power supply signal path induce noise to the amplitude component of the transmission signal, and the noise appears as additional amplitude modulation in the transmission signal after the power-amplification. In modern wireless telecommunication systems using variable-bandwidth transmissions, spurious emissions caused by the noise result in interference between adjacent frequency resource blocks allocated to different communication links and, thereby, reduce the overall capacity of the system. Accordingly, there is a need to reduce the noise power in the power supply signal to obtain more effective power-amplification.